1. Field of the Invention
The present invention relates to a microwave ion source using an ion extraction electrode system with a number of apertures and, more particularly, to a microwave ion source in an ion implanter used in impurity doping, material synthesis, surface modification or new material development.
2. Description of the Prior Art
A conventional large-current ion implanter has an injection ion current of 1 to 10 mA. Semiconductor manufacturing techniques such as SIMOX (Separation by Implanted Oxygen) for forming an SiO.sub.2 layer in a silicon substrate by ion-implanting ions at a dose of 10.sup.18 ions/cm.sup.2 or more have been recently developed. Along with this development, demand has arisen for developing a large-current ion implanter having an ion current of 50 to 100 mA. In order to develop this type of apparatus, a total ion current must be more than 100 to 200 mA (corresponding to an ion current density of 75 to 150 mA/cm.sup.2), and a long lifetime ion source for an active gas such as oxygen is indispensable. It is difficult to obtain such a high-performance ion source even if an ion source used in a conventional ion implanter is improved in performance. For example, ion sources with a thermionic filament are conventionally used since they provide a large ion current density. However, these sources have short lifetime for reactive gases such as oxygen. Therefore, the thermionic filament type ion source cannot provide a practical large-current ion source.
For this reason, a microwave ion source without a filament is expected to be an ion implantion type large-current ion source. However, development and/or study of such an ion source have not substantially be made. No practical applications have been expected for a large-current ion source for, for example, 100 mA ion implanter. For example, in microwave ion sources practically used for ion implanter, as described in U.S. Pat. Nos. 4,058,748 and 4,409,520, a special small discharge space (ridged type, 10.times.40.times.40 mm) is used based on an assumption that high-voltage density cannot be obtained by a large discharge space. With this arrangement, a total ion current is about 30 to 40 mA (corresponding to an ion current density of 40 to 50 mA/cm.sup.2). In order to obtain a higher ion current with the ridged type, fundamental technical improvements must be made.
A microwave ion source for generating a shower-like ion beam is illustrated in, for example, Japanese Patent Application Laid-open No. 55-141729. However, an ion current density of this ion source is as low as 1 mA/cm.sup.2 (corresponding to a total ion current of 80 mA).
No ion source has been proposed wherein long-lifetime and stable operation for a reactive gas are guaranteed, a beam size is about (10 to 20) mm.times.(20 to 50) mm, and a total ion current is about 100 to 200 mA (corresponding to an ion current density of 75 to 150 mA/cm.sup.2). Strong demand has arisen for such large-current ion sources.